1. Technical Field
The present invention relates to a flyback switching power supply, and more particularly to a primary-side regulated switching power supply in which the load state of the secondary side of a transformer is detected on the primary side of the transformer in order to control the output voltage.
2. Background Art
Flyback switching power supplies are typically used in insulated switching power supply applications that require relatively low power ratings on the order of several dozen watts or less. Flyback switching power supplies utilize a transformer with an inverted structure in which the orientations of the primary-side and secondary-side coils or the ways those coils are tapped are inverted. In the transformer, a semiconductor switch is connected in series to the primary-side coil, and power is transmitted from the primary side of the transformer to the secondary side by switching the semiconductor switch. In other words, while the semiconductor switch is ON, current flows through the transformer and energy is stored therein. When the semiconductor switch is switched OFF, the stored energy is output from the secondary-side coil of the transformer across a rectifying diode.
In this type of flyback switching power supply, one well-known method of stabilizing the output voltage from the secondary side is a secondary-side regulated scheme in which fluctuations in the output voltage from the secondary side are detected and fed back to a control circuit on the primary side (see Non-Patent Document 1, for example).
In the secondary-side regulated scheme, fluctuations in the output voltage from the secondary side are typically detected using a shunt regulator, and the detected signals are typically fed back to the control circuit on the primary side using a photocoupler. The control circuit controls the switching of the semiconductor switch according to the detected signals fed back from the secondary side in order to stabilize the output voltage.
During normal operation (which excludes the period of time during which the switching power supply starts up), the auxiliary coil of the transformer supplies power to the control circuit on the primary side. Energy stored in the transformer while the semiconductor switch is ON is transmitted to the auxiliary coil from a terminal with the same polarity as the secondary-side coil when the semiconductor switch is switched OFF, thereby generating a voltage in the auxiliary coil. This voltage is rectified using a diode and smoothed using a capacitor to produce a DC voltage that is used as the supply voltage for the control circuit. More particularly, in the circuit described in Non-Patent Document 1, an inductor is arranged between the diode and the capacitor in order to supply a stable, low-noise supply voltage to the control circuit.
In this secondary-side regulated scheme, the output voltage from the secondary side is detected directly and fed back to the control circuit on the primary side, thereby making it possible to responsively stabilize and control the output voltage of the secondary side.
Meanwhile, another well-known method of stabilizing the output voltage from the secondary side in a switching power supply is a primary-side regulated scheme in which fluctuations in the output voltage from the secondary side are detected on the primary side in order to control the output voltage (see Non-Patent Document 2, for example).
In this primary-side regulated scheme, fluctuations in the output voltage from the secondary side are detected indirectly using the auxiliary coil used to produce a supply voltage for the control circuit, and the control circuit controls the switching of the semiconductor switch according to the detected signals in order to stabilize the output voltage.
In the primary-side regulated circuit disclosed in Non-Patent Document 2, the voltage induced in the auxiliary coil is rectified and smoothed using a diode and a capacitor in order to produce a supply voltage for the control circuit, and the pre-rectified voltage is resistively divided in order to produce a detected voltage of the output voltage of the secondary side, which is also sent to the control circuit. In this way, the auxiliary coil is used both for powering the control circuit and for detecting the output voltage of the secondary side. This makes it possible to remove components such as the shunt regulator and the photocoupler in order to reduce the total number of component parts, thereby making it possible to reduce the cost of the switching power supply.